1. Field of the Invention
This invention relates to a power steering apparatus utilizing a torsion bar.
2. Description of the Prior Art
In a conventional power steering apparatus utilizing a torsion bar, as a factor of feeling from a steering wheel, torsion resistance provided by this torsion bar can be utilized.
When the steering wheel is in its neutral position, there occurs little torsion resistance in the torsion bar, therefore its neutral rigidity becomes weak so that its stability at the time of straight traveling of a vehicle worsens.
Thus, various types of apparatuses for enhancing the neutral rigidity by applying a preset force have been proposed. For example, apparatuses disclosed in Japanese Patent Application Laid-Open No.Hei6-171520 and Hei3-139470 have been well known since before.
In a power steering apparatus disclosed in Japanese Patent Application Laid-Open No.Hei6-171520, an input shaft is inserted into an output shaft so that a space is formed between both the shafts and a ring-shaped spring is incorporated in this space. This spring is formed by cutting one position of the material and can be opened to the right and left from that cut position. Then, pins of the input shaft side and the output shaft side are protruded and the respective pins are inserted into the cut positions of the above spring members.
If the input shaft and the output shaft are rotated relative to each other with the above condition, the ring-shaped spring members are opened from the cut positions. Thus, reaction of the spring acts as a preset force to enhance neutral rigidity.
In a power steering apparatus disclosed in Japanese Patent Application Laid-Open No.Hei3-139470, the input shaft is inserted into the output shaft and protrusions are formed on portions opposing each other of both the shafts. Leaf springs are applied in such a condition that they nip these protrusions.
If both the input shaft and the output shaft are rotated relative to each other, the spring force of the leaf spring acts as a preset force because the phase of those protrusions are contradictory to each other, so that the neutral rigidity is enhanced.
However, in the power steering apparatus disclosed in the above Japanese Patent Application Laid-Open No.Hei6-171520, its preset force is difficult to specify.
For example, if it is intended to increase the preset force, a deflection of the ring-shaped spring member at the initial setting must be increased. However, if the ring-shaped spring member is in largely deflected state, its fitting performance is worsened.
If it is intended to decrease the deflection of the ring-shaped spring member at the time of initial setting in order to improve the fitting performance, the spring constant must be increased in proportional therewith. However, if the spring constant is increased, stress in the spring is increased in proportional therewith. Thus, there sometimes occurs a case in which the spring is destroyed even in ordinary application range.
In a power steering apparatus disclosed in Japanese Patent Application Laid-Open No.Hei3-139470, if the protrusions on both the shafts are not set symmetrically, the preset forces are dispersed so that looseness may occur. If it is intended to arrange them symmetrically to prevent such a phenomenon, quite high an accuracy is needed and therefore productivity of this product drops.